1. Field of the Invention
The present invention relates to a rubber composition and a pneumatic tire containing the rubber composition used in at least a part of the tire.
2. Related Art
According to an increasing demand of reducing fuel consumption of automobiles in recent years, there is a strong demand of decreasing the rolling resistance of tires. It has been known that the rolling resistance relates to heat generation of a rubber composition, and decrease of a hysteresis loss of rubber, i.e., suppression of a loss coefficient (tan δ) of a rubber composition to a low level, is effective for decreasing the rolling resistance.
Various proposals have been made as a technique for suppressing heat generation of a rubber composition, and for example, JP-A-2005-146076 discloses a rubber composition for a side tread that contains 100 parts by weight of vulcanizable rubber containing 65% by weight or more of natural rubber or polybutadiene rubber, from 30 to 80 parts by weight in total of silica and/or carbon black having a nitrogen absorption specific surface area (N2SA) of from 20 to 85 m2/g, and from 0.1 to 10 parts by weight of a specific cyclic polysulfide, thereby having high hardness and high strength and elongation, and being suppressed in increase of tan δ.
In recent years, an ethanol fuel is receiving attention as a fuel for automobiles, and the demanded amount thereof is being increased quickly. Various studies have been made for reuse of bagasse, which is a squeezed residue of sugarcane as a raw material of an ethanol fuel. For example, there are studies of using bagasse as a soil ameliorant, an energy source for factories, livestock feed, and raw material of paper.
Charcoal is produced with bagasse in recent years. Bagasse charcoal is suitable for growth of microorganisms, such as phosphate-solubilizing bacteria, owing to the porous structure thereof, and has high water retention capability. Accordingly, bagasse charcoal is intended to be used as a soil ameliorant of farmland for enhancing yields and qualities of crops.
The use of porous particles mixed in a rubber composition for a tire has been disclosed in literatures. For example, JP-A-2000-211315 discloses a rubber composition for a tread in which from 2 to 30 parts by weight, based on 100 parts by weight of a rubber base material, of porous particles (activated charcoal) having an average particle diameter of 20 μm or less and a specific surface area of 800 m2/g or more are mixed, JP-A-11-269306 discloses a rubber composition for a winter tire in which from 1 to 10 parts by weight of activated carbon particles having an average particle diameter of from 30 to 500 μm and the same weight parts as the activated carbon or less of powdered vulcanized rubber having an average particle diameter of from 50 to 1,000 μm are mixed based on 100 parts by weight of rubber, and JP-A-2005-162865 discloses a rubber composition in which from 1 to 20 parts by weight, based on 100 parts by weight of a diene rubber component, of particles containing a vegetable porous carbide (bamboo charcoal) are mixed. All the rubber compositions are disclosed that they improve performance of tires on ice and snow, particularly frictional force on ice. However, it has not yet known that the bagasse charcoal having porous nature is mixed in a rubber composition.
A rubber composition for a tire is demanded to cause less heat generation and to have high reinforcement property for reducing rolling resistance and ensuring durability of the tire. According to the demands, such a method has been conventionally studied that SBR and BR are mixed with a formulation containing mainly natural rubber for reducing tan δ of the rubber composition as less as possible, whereby heat generation of the rubber composition itself is suppressed. Upon increasing the ratio of natural rubber, however, the breaking strength is improved, but there is a tendency that the low heat generating property cannot be obtained, and thus it is difficult to attain both low heat generating property and high reinforcement property simultaneously at high levels.
The rubber composition is also demanded to be excellent in workability including rolling workability and extrusion workability and be good in surface property of a molded article. Upon increasing the addition amount of the reinforcing filler, however, there is a tendency that the workability and the surface property are deteriorated.